One type of known humidity sensor comprises a capacitor having a dielectric constant which changes as a function of humidity. Such capacitance humidity sensors have been in the form of a dielectric layer composed of a polymer such as polyimide and a thin metal electrode conducting layer, often made of gold. See Chen U.S. Pat. No. 4,761,710 issued Aug. 2, 1988, Abadie et al. U.S. Pat. No. 4,603,372 issued July 29, 1986, Kuisma et al. U.S. Pat. No. 4,500,940 issued Feb. 19, 1985, Chambaz et al. U.S. Pat. No. 4,438,480 issued Mar. 20, 1984, Heywang et al. U.S. Pat. No. 4,305,112 issued Dec. 8, 1981, Nelson U.S. Pat. No. 4,345,301, issued Aug. 17, 1982, Mills U.S. Pat. No. 4,337,658, issued July 6, 1982, and Suntola U.S. Pat. No. 4,164,868 issued Aug. 21, 1979. If such devices are to function well, at least one electrode must be permeable to water, have a low electrical resistance, and be relatively insensitive to corrosion. When an ultra thin gold electrode is used, good electrical conductivity and good permeability can be achieved. However, such capacitors have poor corrosion resistance. The thin gold electrode can be rapidly destroyed by sulfur-based pollutants or chlorine in the air surrounding a swimming pool.
Polyimide is a particularly useful dielectric for such sensors because its dielectric constant is linearly proportional to its moisture content. Furthermore, the excellent thermal resistance of polyimide makes it useful in capacitance humidity sensing devices. However, the bonding between the polyimide and metal electrode layers is difficult to obtain without the use of adhesives because of the dissimilarity between the metal and plastic.
Conductive compositions comprising conductive particles, such as particles of silver or carbon black, dispersed in resins such as polyimide, are generally known. See, for example, Takenaka U.S. Pat. No. 3,697,450, issued Oct. 10, 1972, describing resistance films. Other known humidity sensors have employed successive layers of cross-linked polymeric resin materials such as cellulose acetate butyrate cross-linked with urea formaldehyde resin. In one such sensor, a crosslinked cellulose acetate butyrate core containing conductive particles such as carbon is sandwiched between a pair of outer resin layers free of carbon particles. See Thoma U.S. Pat. No. 3,458,845, issued July 29, 1969. In other capacitive humidity sensors, the outer resin layers contain the conductive particles, and the inner resin layer does not; see Thoma U.S. Pat. Nos. 3,582,728, issued June 1, 1971, 3,802,268, issued Apr. 9, 1974, and IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-2, No. 3, 1979, pages 321-323. Baxter et al. U.S. Pat. No. 4,564,882, issued Jan. 14, 1986, describes a humidity sensing element wherein the dielectric layer can be made from either cellulose acetate butyrate or polyimide.
Polyparabanic acids are known polymers used in a variety of applications. These polymers are generally defined as: ##STR1## wherein R is an organic moiety which may be aromatic, aliphatic or alicyclic. See Henderson et al., "Poly(parabanic) Acids-A New Family of Thermoplastics" pp. 660-674. Poly(iminoimidazolidinediones) and other heterocyclic polymers related to PBA in structure are also known, and have been used to make films. See, for example, Patton U.S. Pat. Nos. 3,547,897, issued Dec. 15, 1970 and 4,105,616, issued Aug. 8, 1978, Johnson et al. U.S. Pat. No. 3,939,116, issued Feb. 17, 1976, and Polymer Preprints, Vol. 12, No. 1, Mar. 1971, pp. 162-169. Hawkins U.S. Pat. No. 4,332,976, issued June 1, 1982, describes PBA tape used in coaxial cables.
Screen printing has been suggested as a method for forming certain types of layers in humidity sensors. Mills U.S. Pat. No. 4,298,855, issued Nov. 3, 1981, which describes forming electrical resistors comprising carbon particles dispersed in a polymer film by such a process. Djorup U.S. Pat. No. 4,793,182, issued Dec. 27, 1988, describes a constant temperature hygrometer wherein resistive conductors are formed by silk screen printing.
The present invention addresses the various drawbacks with known capacitance humidity sensors discussed above, and provides a humidity sensor having a number of unexpected superior characteristics.